228 ANIMAL PROTEINS 



is indeed an essential element in the success of the 

 process. 



The greases obtained, whether by the use of fat solvents 

 or by skimming off during extraction, or in any other way, 

 are mixed together as is appropriate to their origin and 

 purity, and subjected to further puiification, the object of 

 which is to remove gelatinous and albuminous matters, and 

 to decompose lime or soda soaps. The precise methods of 

 purification are, of course, dependent mainly upon the 

 impurities known to be present, but the readiest method is 

 to give the grease further steaming or boiling with water, 

 and so effect by washing and by solvent action the elimination 

 of non-fatty matters. In many cases it is found advan- 

 tageous to employ mineral acids or oxidising agents to assist 

 the process. The process may be repeated as often as is 

 desired. 



The recovered and purified greases are often of a high 

 standard of purity, and the best are quite fit for edible pur- 

 poses. The large extension of the margarine industry in this 

 country has indeed caused a larger proportion than ever of this 

 bye-product to be so used. In some cases it is found com- 

 mercially advantageous to submit the grease to action of the 

 filter press, and so to separate it into solid and liquid portions, 

 the former containing a much larger proportion of stearin, 

 and the latter of olein. Much of the grease from the gelatine 

 trade is also found suitable for soap manufacture, and is 

 therefore a valuable source of glycerine. 



Other Raw Materials. Whilst hide pieces and flesh- 

 ings, and animal bones, comprise the principal raw material 

 for the manufacture of gelatine and glue, there are also 

 minor sources of raw material which, though often not 

 suitable for gelatine manufacture, will yield a satisfactory 

 glue. Thus the skins, bladders and bones of fish form the 

 source of " fish glue." Sole skins, indeed, when deodorized 

 by chlorine and decolorized by animal charcoal, are 

 made into gelatine. The bladders of some fish (e.g. the 

 sturgeon) are washed, purified and dried with rolling to 

 make " isinglass," a form of natural gelatine in which 



